This invention relates to a method for producing a porous cross-linked polymer sheet of an extremely small water content by dehydrating with a non-woven fabric roll a porous cross-linked polymer of a prescribed thickness obtained from a water-in-oil type high internal phase emulsion (hereinafter occasionally referred to simply as xe2x80x9cHIPExe2x80x9d).
As a technique for obtaining a porous substance formed of uniform open cells of a minute diameter, a method for producing a polymer by cross-link polymerizing an HIPE in the presence of a specific surfactant has been known. It is generally held that the term xe2x80x9cHIPExe2x80x9d used herein refers to an emulsion so formed that the disperse phase accounts for a ratio exceeding 70 vol. % based on the total volume thereof [K. J. Lissant, Journal of Colloid and Interface Science, Vol. 22, 462 (1966)]. U.S. Pat. No. 5,334,621, for example, discloses a method for producing a porous cross-linked polymer by using such an HIPE (hereinafter referred to simply as xe2x80x9cHIPE methodxe2x80x9d).
This HIPE method produces a porous cross-linked polymer by preparing an HIPE containing (i) a polymerizing monomer mixture containing an oil-soluble vinyl monomer and a cross-linked monomer possessed of not less than two functional groups in the molecular unit thereof, (ii) a water phase accounting for 90 mass %, preferably 95 mass %, and particularly preferably 97 mass % of the emulsion, (iii) a surfactant such as a sorbitan fatty acid ester and a glycerol mono-fatty acid ester, and (v) a polymerization initiator and heating the HIPE till it polymerizes and forms a cross-link. According to this HIPE method, a porous cross-linked polymer formed of open cells in a reticular pattern is produced by virtue of reversed-phase emulsion polymerization. The porous cross-linked polymer which is obtained by the HIPE method, therefore, is possessed of such characteristic properties as low density, water absorbing property, water retaining property, heat insulation and soundproofing property.
The porous cross-linked polymer which is produced by the HIPE method described above, however, is disposed during the process of production to expose the formation of cells to the influence of the ratio of the water phase, i.e. an internal phase to the oil phase, i.e. an external phase, namely W/O, during the reversed-phase emulsion polymerization. An effort to obtain a porous cross-linked polymer having as large a void volume ratio as permissible inevitably results in increasing the water phase side numeral of the ratio W/O. The porous cross-linked polymer of this quality enjoys a large demand. The reason for this large demand is that when the porous cross-linked polymer has a large void volume ratio, it excels not only in absorbing property but also in heat insulating property and sound insulating property and, therefore, finds utility in various fields such as building materials, audio products, and horticultural articles. U.S. Pat. No. 5,334,621 mentioned above, for example, has a statement that the porous cross-linked polymer, when produced by the HIPE method, particularly preferably has a density of 97 mass % (W/O=33/1). Even generally, the W/O is actually attained more often than not in the range of 30/1-100/1. Specifically, an attempt to produce a porous cross-linked polymer by the HIPE method necessitates a large volume of water for the formation of an HIPE. This fact implies that when the porous cross-linked polymer is produced by the HIPE method, the produced porous cross-linked polymer is fated to contain water and, therefore, is required to be dehydrated and dried.
The dehydration of the porous cross-linked polymer which has been obtained by the polymerization of an HIPE is effected, as demonstrated in Example 2 cited in the official gazette of National Unexamined Patent Publication 2000-500,796, by nipping the polymer between paper towels and slowly squeezing it till the aqueous phase is removed.
The official gazette of National Unexamined Patent Publication HEI-11-503,177 discloses the dehydration which is effected by compressing an HIPE foam to expel the residual water therefrom or subjecting the foam and the water lodged therein together to a temperature in the approximate range of 60xc2x0 C. to 220xc2x0 C. or a microwave treatment, to vacuum dehydration, or to the combination of compression and thermal drying/microwave/vacuum dehydration. This compressive dehydration is accomplished by the compression which is produced with a series of paired porous nip rolls provided with a vacuum unit adapted to decrease the amount of the residual water phase to about three times the mass of the monomer which has been polymerized.
In the official gazette of WO86/06,766 is disclosed a liquid absorbing device which is used for removing a liquid from a given object for treatment containing the liquid. Specifically, this device is formed by wrapping a felt of excellent initial liquid absorbing power around the surface of a liquid absorbing roll and combining the liquid absorbing roll with an aspiration mechanism and is consequently enabled to utilize effectively the capillary effect originating in the numerous pores inherently possessed by a fibrous sheet, smooth the transfer of liquid, and exalt the liquid absorbing power and the property of retaining the liquid absorbing power.
The porous cross-linked polymer obtained by polymerizing an HIPE, however, has a high water content and is deficient in strength. When the foam and the porous cross-linked polymer obtained by polymerizing an HIPE which are disclosed in the official gazette of National Unexamined Patent Publication 2,000-500,796 and HEI-11-503,177 are dehydrated, therefore, they cannot be fully satisfactorily dehydrated or, when compelled to be sufficiently dehydrated, they possibly encounter fracture during the course of dehydration. The porous cross-linked polymer obtained by polymerizing an HIPE, though depending on the W/O ratio of the HIPE to be used, has a water content reaching a level in the range of 300-25,000 (w/w)% based on the mass of the polymer and, therefore, is particularly deficient in mechanical strength. Once the compressing operation of a metal roll inflicts a crack on the surface of the product for contact with the roll, this crack will form a cause for lowering the rating of the product in appearance. Further, when the product happens to sustain a crack or a bend, for example, this damage will form a cause for degrading the water absorbing property which the product is inherently required to possess and will possibly induce degradation of the quality of the product in terms of function.
Further, for the purpose of allowing a salt capable of stabilizing emulsification during the formation of an HIPE to be incorporated in the water phase thereby enabling the emulsification to proceed smoothly, such an electrolyte as calcium chloride is added to the emulsion. When the dehydration resorts solely to the use of a porous nip roll furnished with an aspiration mechanism, therefore, the roll is suffered to gather dirt on the surface thereof and the pores in the roll are clogged with dirt possibly to the extent of degrading the capacity of dehydration and interrupting the operation of dehydration because the sparingly soluble precipitate and the unaltered monomer are also subjected to dehydration. This interruption of the dehydrating operation forms a cause for inducing degradation of the efficiency of production particularly when the porous cross-linked polymer is continuously produced at a high speed.
An object of this invention, therefore, is to provide a method for causing a porous cross-linked polymer obtained by polymerizing an HIPE to be dehydrated without inflicting any damage on the polymer and a method for producing a porous cross-linked polymer sheet allowing a dehydrating treatment to be performed continuously and smoothly thereon.
This invention has been perfected based on the discovery that when a porous cross-linked polymer obtained by polymerizing an HIPE is dehydrated by a treatment using to a non-woven fabric roll and an aspiration mechanism in combination, a polymer sheet free from such damage as fold and crack can be produced with very high efficiency.
Specifically, this invention is accomplished by the following item (1).
(1) A method for the production of a porous cross-linked polymer sheet, comprising the steps of forming and polymerizing an HIPE thereby obtaining a porous cross-linked polymer and dehydrating the porous cross-linked polymer by using a non-woven fabric roll furnished with an aspiration mechanism.
In accordance with this invention, the use of non-woven fabric rolls furnished with an aspiration mechanism enables a porous cross-linked polymer to be dehydrated to an extremely low water content. In order for the conventional method to acquire this low water content, it generally necessitates an increase in the linear pressure of roll and consequently induces the porous cross-linked polymer to sustain fracture. In this invention, however, by particularly specifying the thickness of the porous cross-linked polymer, it is made possible to dehydrate the polymer sheet thoroughly to the center thereof with unusually high efficiency.